Gas turbine engines are often produced to include turbine nozzles, which meter combustive gas flow while also accelerating and turning the gas flow toward the blades of a turbine rotor positioned downstream of the nozzle. A turbine nozzle may be assembled from a number of separately-produced segments, which each include an arced segment of the inner endwall or ring, an arced segment of the outer endwall or ring, and one or more airfoils or vanes extending therebetween. Turbine nozzles of this type (referred to herein as “segmented turbine nozzles”) may, however, be prone to gas flow leakage across the mating interfaces between adjoining segments and flange surfaces. Gas flow leakage may increase chargeable cooling flow to the cycle, reduce cooling flow from the combustor, and result in a direct penalty against overall gas turbine engine performance. As an alternative to segmented turbine nozzles, bi-cast turbine nozzles have been developed in which inner and outer rings are cast as unitary structures, which lack segment-to-segment interfaces across which leakage may occur. During production of a bi-cast turbine nozzle, the vanes are individually produced and then positioned in a mold for bonding to the inner ring and/or the outer ring. In certain cases, the vanes may only be affixed to one of the rings, while forming slip joints with the ring to which the vanes are not affixed. Such slip joints help alleviate thermomechanical fatigue by permitting relative radial movement between the turbine nozzle vanes and the turbine nozzle attachment points as thermal gradients develop across the turbine nozzle during operation of the gas turbine engine.
While providing the above-noted advantages, bi-cast turbine nozzles remain limited in certain respects. For example, by virtue of the manner in which the bi-cast turbine nozzles are produced, the application of the Thermal Barrier Coating (TBC) typically occurs after assembly of the turbine nozzle and bonding of the vanes. When the turbine nozzle is produced to include slip joints, the interior surfaces defining the slip joints may remain uncoated by the TBC. Unfilled clearances or air gaps may thus remain around the floating terminal ends of the vanes bounding the slip joints. During gas turbine engine operation, hot combustive gasses may be ingested into these gaps from the pressure side of the vane, flow underneath the vane footprint, and discharge to the suction side of the vane. Hot gas ingestion may cause oxidative damage to the slip joint surfaces and detract from the aerodynamic performance of the turbine nozzle. Over the lifespan of the turbine nozzle, oxidation and hot corrosion may further erode the uncoated surfaces of the slip joints resulting in increasingly large air gaps and still further reductions in the aerodynamic performance of the turbine nozzle.
It is thus desirable to provide embodiments of a method for manufacturing turbine nozzles including slip joints, which minimize hot gas ingestion into the slip joints and which provide oxidation and hot corrosion resistance during operation of the gas turbine engine. It would also be desirable if, in at least some embodiments, the manufacturing method enabled the production of bi-cast turbine nozzles; that is, turbine nozzles wherein the inner and outer rings are separately cast as unitary structures. It would also be desirable to provide embodiments of a turbine nozzle produced in accordance with such a fabrication method. Other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description and the appended Claims, taken in conjunction with the accompanying Drawings and the foregoing Background.